Introduction to particle physics

1. Introduction to particle physics Hal

2. Particle physics want to answer • What are the “elementary” constituents of Matter? • What are the forces that control their behaviour at the most basic level?

3. What is elementary means ? • The word “elementary” is used in the sense that such particles have no known structure, they are “pointlike. ” • “elementary” depends on the spatial resolution of the probe used to investigate possible structure.

4. Units in particle physics

5. History of Constituents of Matter

6. History of Constituents of Matter Дми́трий Ива́нович Менделе́ев

7. History of Constituents of Matter Electrons were first discovered as the constituents of cathode rays. In 1897 British physicist J. J. Thompson showed the rays were composed of a previously unknown negatively charged particle, which was named electron.

8. fluorescent screen a - particles radioactive source target (very thin Gold foil) Detector (human eye) History of Constituents of Matter

9. a - particle Atom: spherical distribution of electric charges impact parameter b Proton discovery For Thomson’s atomic model the electric charge “seen” by the a – particle is zero, independent of impact parameter no significant scattering at large angles is expected An atom consists of a positively charged nucleus surrounded by a cloud of electrons

10. History of Constituents of Matter James Chadwick's 1932 discovery of the neutron Neutron

11. scattered neutron (not visible) incident neutron (not visible) recoil nucleus (visible by ionization) Neutron discovery Neutron discovery: observation and measurement of nuclear recoils in an “expansion chamber” filled with Nitrogen at atmospheric pressure

12. Incident neutron direction Plate containing free hydrogen (paraffin wax) Recoiling Nitrogen nuclei proton tracks ejected from paraffin wax From non-relativistic energy-momentum conservation mp: proton mass;mN: Nitrogen nucleus mass 2m m + mp 2m m + mN Up = Vmax UN = Vmax Upm + mN UNm + mp = Neutron discovery Assume that incident neutral radiation consists of particles of mass m moving with velocities v < Vmax Determine max. velocity of recoil protons (Up) and Nitrogen nuclei (UN) from max. observed range From measured ratio Up / UN and known values of mp, mN determine neutron mass: m mn mp Present mass values : mp = 938.272MeV/c2; mn = 939.565MeV/c2

13. P.A.M. Dirac electron spin electron magnetic dipole moment me Antimatter Dirac equation two solutions 1928 For each solution of Dirac’s equation with electron energy E> 0 there is another solution with E< 0 What is the physical meaning of these “negative energy” solutions ? Motion of an electron in an electromagnetic field: presence of a term describing (for slow electrons) the potential energy of a magnetic dipole moment in a magnetic field existence of an intrinsic electron magnetic dipole moment opposite to spin

14. 23 MeV positron 6 mm thick Pb plate Carl D. Anderson 63 MeV positron Cosmic-ray “shower” containing several e+ e– pairs Antimatter discovery Cloud chamber photograph by C.D. Anderson of the first positron ever identified.The positron must have come from below since the upper track is bent more strongly in the magnetic field indicating a lower energy 1932

15. Neutrinos A puzzle in b – decay: the continuous electron energy spectrum (Two body decay) electron total energy E = [M(A, Z) – M(A, Z+1)]c2 First measurement by Chadwick (1914) Radium E: 210Bi83 (a radioactive isotope produced in the decay chain of 238U) b- decay: n  p + e- + n (E. Fermi, 1932-33)

16. First neutrino detection n + p  e+ + n 2 m H2O + CdCl2 I, II, III: Liquid scintillator (Reines, Cowan 1953) detect 0.5MeVg-rays from e+e– g g(t= 0) Eg = 0.5MeV neutron “thermalization” followed by capture in Cd nuclei  emission of delayed g-rays (average delay ~30ms) Event rate at the Savannah River nuclear power plant: 3.0  0.2 events / hour (after subracting event rate measured with reactor OFF ) in agreement with expectations

17. History of Constituents of Matter Late 1950’s – early 1960’s: discovery of many particles at the high energy proton accelerators (Berkeley Bevatron, BNL AGS, CERN PS), all with very short mean life times (10–20 – 10–23s,collectively named “hadrons”) ARE HADRONS ELEMENTARY PARTICLES?

18. History of Constituents of Matter 1964 (Gell-Mann, Zweig): Hadron classification into “families”; observation that all hadrons could be built from three spin ½ “building blocks.” (named “quarks” by Gell-Mann) The three quarks are u(+2/3),d(-1/3),s(-1/3). And three antiquarks ( u , d , s ) with opposite electric charge.

19. Mesons: quark – antiquark pairs Examples of non-strange mesons: Examples of strange mesons: Baryons: three quarks bound together Antibaryons: three antiquarks bound together Examples of non-strange baryons: Examples of strangeness –1 baryons: Examples of strangeness –2 baryons:

20. What are the “elementary” constituents of Matter? 3 x 6 = 18 quarks + 6 leptons = 24 fermions (constituents of matter) + 24 antiparticles 48 elementary particles consistent with point-like dimensions within the resolving power of present instrumentation ( ~ 10-16 cm)

21. What are the forces that control their behaviour at the most basic level? Electromagnetic interaction (all charged particles) Infinite interaction radius Gravitational interaction (all particles) Totally negligible in particle physics

22. scattered electron ( Ee , p’ ) incident electron ( Ee , p ) q Energy – momentum conservation: Eg = 0 pg = p – p ’ ( |p| = |p ’| ) g incident proton ( Ep ,– p ) scattered proton ( Ep ,– p’ ) No static fields of forces In Relativistic Quantum Mechanics static fields of forces DO NOT EXIST ; the interaction between two particles is “transmitted” by intermediate particles acting as “interaction carriers” Example: electron – proton scattering (an effect of the electromagnetic interaction) is described as a two-step process : 1.incident electron  scattered electron + photon 2.photon + incident proton  scattered proton The photon ( g ) is the carrier of the electromagnetic interaction • “Mass” of the intermediate photon:Q2 Eg2 – pg2 c2 = – 2p2 c2 ( 1 – cosq ) • The photon is in a VIRTUAL state because for real photons Eg2 – pg2 c2 = 0 • (the mass of real photons is ZERO ) – virtual photons can only travel over • very short distances thanks to the “Uncertainty Principle”

23. Weak interaction The weak interaction is the only force affecting neutrinos (except for gravitation). Its most familiar effect is beta decay. The weak interaction is unique in a number of respects: 1.It is the only interaction capable of changing flavour. 2.It is the only interaction which violates parity symmetry P (because it almost exclusively acts on left-handed particles). 3.It is mediated by massive gauge bosons.

24. Strong interaction In particle physics, the strong interaction, or strong force, or color force, holds quarks and gluons together to form protons, neutrons, baryons and mesons. The interaction radius 10 –13 cm. The theory about strong force is quantum chromodynamics (QCD). Each quark exists in three states of a new quantum number named “colour” Particles with colour interact strongly through the exchange of spin 1 particles named “gluons”, in analogy with electrically charged particles interacting electromagnetically through the exchange of spin 1 photons.

25. Strong interaction Free quarks, gluons have never been observed experimentally; only indirect evidence from the study of hadrons – WHY? CONFINEMENT: coloured particles are confined within colourless hadrons because of the behaviour of the colour forces at large distances The attractive force between coloured particles increases with distance  increase of potential energy production of quark – antiquark pairs which neutralize colour  formation of colourless hadrons (hadronization)

26. END

29. Conclusion

30. 1937: Theory of nuclear forces mc2200 MeV for Rint10 -13 cm the muon is not Yukawa’s meson (H. Yukawa) Existence of a new light particle (“meson”) as the carrier of nuclear forces Relation between interaction radius and meson mass m: Hideki Yukawa Yukawa’s meson initially identified with the muon – in this case m– stopping in matter should be immediately absorbed by nuclei  nuclear breakup (not true for stopping m+because of Coulomb repulsion - m+never come close enough to nuclei, while m–form “muonic” atoms) Experiment of Conversi, Pancini, Piccioni (Rome, 1945): study of m–stopping in matter using m– magnetic selection in the cosmic rays In light material (Z  10) the m– decays mainly to electron (just as m+) In heavier material, the m– disappears partly by decaying to electron, and partly by nuclear capture (process later understood as m– + p  n + n). However, the rate of nuclear captures is consistent with the weak interaction.

31. 1947: Discovery of the p - meson Four events showing the decay of a p+ coming to rest in nuclear emulsion mp = 139.57 MeV/c2 ; spin = 0 Dominant decay mode: p+m+ + n (and p –m– + n ) Mean life at rest: tp= 2.6 x 10-8s = 26 ns (the “real” Yukawa particle) Observation of the p+m+ e+ decay chain in nuclear emulsion exposed to cosmic rays at high altitudes Nuclear emulsion: a detector sensitive to ionization with ~1mm space resolution (AgBr microcrystals suspended in gelatin) In all events the muon has a fixed kinetic energy (4.1 MeV, corresponding to a range of ~600mm in nuclear emulsion)  two-body decay p – at rest undergoes nuclear capture, as expected for the Yukawa particle A neutral p – meson (p°) also exists: m (p°) = 134. 98 MeV /c2 Decay: p° g + g, mean life = 8.4x10-17 s p – mesons are the most copiously produced particles in proton – proton and proton – nucleus collisions at high energies

34. CONCLUSIONS The elementary particles today: 3 x 6 = 18 quarks + 6 leptons = 24fermions (constituents of matter) + 24 antiparticles 48 elementary particles consistent with point-like dimensions within the resolving power of present instrumentation ( ~ 10-16 cm) 12 force carriers (g, W±, Z, 8 gluons) + the Higgs spin 0 particle (NOT YET DISCOVERED) responsible for generating the masses of all particles

35. Cosmic rays underground • At the surface, muons contribute more than a half to the total cosmic ray flux. • The energy spectrum of muons: dN / dE E -3.7 (at E>1 TeV). • Only muons and neutrinos can penetrate to large depths underground. • The background from cosmic-ray muons for underground experiments will be considered later in the course. • Atmospheric neutrinos are hard to detect due to small interaction cross-section. Nevertheless their flux has been measured and the deficit of muon neutrinos has been observed pointing to the neutrino oscillations - this will be considered in detail later. Experimental Astroparticle Physics

36. Cosmic rays underground • Review of Particle Physics: pdg.lbl.gov (Cosmic Rays). • Muon flux as a function of depth underground: x (m w. e.) = depth (m)   (g/cm3) • Neutrino-induced muons dominate at x > 15 km w. e. Experimental Astroparticle Physics

37. Energy loss of muons Experimental Astroparticle Physics

38. Particle Zoo

39. Thomson (1897): Discovers electron

40. An old gaseous detector based on an expanding vapour; ionization acts as seed for the formation of liquid drops. Tracks can be photographed as strings of droplets scattered neutron (not visible) incident neutron (not visible) recoil nucleus (visible by ionization) Neutron discovery Neutron discovery: observation and measurement of nuclear recoils in an “expansion chamber” filled with Nitrogen at atmospheric pressure

41. fluorescent screen a - particles radioactive source target (very thin Gold foil) Detector (human eye) Example: Rutherford’s scattering ~ resolving power of Rutherford’s experiment a-particle mass 0.05c